A method of fabricating a photodiode having a cutoff wavelength of 2.39 μm is shown in Non-patent Literature 1. A photo detector has an absorption layer disposed on an InP substrate, and a p-type InGaAs window layer. The absorption layer has a type-II quantum well structure of InGaAs/GaAsSb. After mesa etching, a SiO2 passivation layer is formed on the p-type InGaAs window layer.